If it had to be a sad ending, I'd have made it so that he killed his son and the other two, then went outside and was killed by a monster. Or at the very least some time should have elapsed before the military arrived and made everything ok. That ending, where we see the mother who left at the start alive and well, strongly suggested that Carmody and her nutjob crew were right and not only that; they'd be saved in a couple of minutes. ARGH.